[Moon-net] Dynamic range

[Leif Asbrink]

Hello Kim,

> In case you want good
> dynamic range you can use an SDR-IQ for USD 499. It
> will also give you one channel only so you will not be
> able to take full advantage of X-pol antennas.
> 
> Whatever for a hi dynamic range capability, the used mixer are capable of 
> handling, I still do find the purity of the used LO as the main problem 
Hmmm, the SDR-IQ samples at 66.666 MHz using an X-tal oscillator
so such problems are not present.

> second its ALWAYS the mixer, who colaps.- so IF we use a watkins-Johnson 
> mixer   (1W of LO), I would state, that the problem will show, to be a lack 
> of purity of the LO -signal.
To use the SDR-IQ you just need an X-tal oscillator to convert
to somewhere below 30 MHz and purity of LO is no problem at all.

What really sets the limit is the poor quality on the tx side from
our friends who use conventional transceivers!

"Theoretically" the SDR-IQ has its noise floor 74 dB below full scale 
at a bandwidth of 33.333 MHz (assuming it has the same front end as the
SDR-14.) That is about -149 dBc/Hz. To get rid of false signals at
the IM3 , IM5,... frequencies one has to place the noise floor
from the preamp about 40 dB above this internal noise floor which
means that the SDR-IQ will provide -110 dBc/Hz when operated in
the totally "IM3-free mode". Usually it will be enough to place
the noise floor at -125 dBc/Hz. When several signals are present
they will function as dithering for each other and remove the
"IM3" signals.

> Whatever the now-adays DDS used, I think, that these LO´s NOT is clean 
> enough, and can not compete with a XO
> chain designed properly - so, I havent a solution for the problem Leif, but 
> have indeed often been thinking of, how you guys managed to work EME in the 
> Stockholm area.  High powerlevels, large antennas etc.
> 
> Whatever the soundcard DO have a good dynamic range - well, what the 
> tranceivers used nowadays ??
> 
> I dont have a solution on this issue - do you ??

Conventional transceivers are like this in tx mode:
               Noise floor in -dBc/Hz  
           5kHz  10kHz  15kHz  20kHz  50kHz 
FT-736    115.7  123.7  126.7  128.4  130.8
FT-817    101.7  110.6  114.8  118.0  126.7
FT-847     94.3  107.3  112.7  116.1  125.2
IC821H     95.8  105.1   -     113.1  122.1
IC910H     96.9  106.2  111.1  113.7  121.3
IC970H    100.1  111.9   -     121.6  130.0  
TS-2000   105.3  115.3  119.8  122.6  131.0
(Steady carrier)

Very soon I expect someone to make a digital transmitter. It is 
difficult to avoid spurs when starting with a soundcard, but I 
expect a big improvement in spectral purity as soon as we can 
generate RF signals directly from a D/A converter at 1MHz or 
higher. Then up-conversion with X-tal oscillators will become 
easy:-)

Right now I am working on tx software, but not having the target 
hardware I use modified WSE RX converters. There are a couple of 
spurs, but the noise floor is really low:-)

73

Leif / SM5BSZ